3.3 Physical and Chemical Changes

In physical changes:

The appearance of a substance changes, but the chemical bonds holding the substance together do not change.

No new substance is formed ( it doesn’t change the identity of the substance)

It is also fairly easy to reverse.

Examples: melting, freezing, boiling

Water, whether in the form of liquid or ice is still H2O. The chemical formula has not changed even if the appearance is different.

See page 98

In chemical changes:

New substances are produced in the process of breaking chemical bonds and forming new ones.

They are difficult to reverse.ex: cooking is a chemical change

can’t reserve an omelet egg back into original egg!!!

Evidence of chemical change:

a) Colour changeb) Heat, light, sound produced or consumedc) Bubbles of gas formd) Formation of a precipitatee) The change is difficult to reverse

Interesting exampleRotting ApplesWhen apples go bad (rot) this is a chemical change, scientists find that many new chemical substances are produced, such as a gas called “ethene” which is a chemical messenger that signals to other neighboring apples to start the process of rotting too!

So one bad apple could spoil the bunch !

This is both a chemical and clearly a physical change

Can you guess if it is endothermic or exothermic?

Energy Changes in chemical reactions

In both physical and chemical changes, energy changestake place.

Energy can either be released/given off or absorbed/taken in.

Depending on the energy changes, we can have two types of reactions:

Two types of reactions

Exothermic reactions involve the overall ___________ of energy in the form of heat and light.

Energy exits

Endothermic reactions involve the overall ___________of energy

Energy enters

Campfire: exothermicInstant Cold Pack: Endothermic

Applications of Chemical Changes

Some chemical changes present problems, while others provide opportunities and advantages

Corrosion(rust): is a major problem for steel structures - by protecting or coating steel surfaces with a protective layer, the chemical reaction of iron with oxygen/water can be prevented.

First nations people of the Pacific Coast have used smokingas a means of preserving food. Smoke causes chemicalchanges in meat that kill bacteria.

Homework

Page 99

Questions 2 and 3

Congrats! The Chemistry part of Science 9 is done